This invention relates to a power transmission belt and a method of producing the power transmission belt, and in particular relates to measures for abnormal vibration and noise production in the case that the belt is run in serpentine drive.
There is known a power transmission belt in which a top fabric is integrally adhered with the upper surface of an adhesion rubber layer. Such a power transmission belt is generally shaped like a cylinder by lap joint that both ends of the top fabric like a band are overlapped so as to put one on another and are jointed together. Therefore, the lap-jointed portion produces a level difference.
In the case of forming a power transmission belt using the top fabric in which a level difference is produced, as shown in FIG. 7 (Prior Art 1), if the top fabric b is engaged with the outer periphery of a molding drum a and is wrapped in an adhesion rubber sheet c made of an unvulcanized rubber sheet, a cord d, another adhesion rubber sheet e made of an unvulcanized rubber sheet, and a rib rubber sheet f made of an unvulcanized rubber sheet in this order, the cord d may be displaced in level in correspondence with the level difference of a lap-jointed portion h of the top fabric b. Then, such a substance at this stage is press cured with a mold thereby forming a cylinder-shaped molded substance. The molded substance is then removed from the mold and is cut in a specific width thereby obtaining a power transmission belt. In the obtained power transmission belt, the cord d is embedded into the adhesion rubber layer with deviated from an adequate position.
If the power transmission belt mentioned above is used in serpentine drive that the belt is looped around a pulley so as to contact the belt back face with the pulley, the running of the belt becomes unsmooth. In detail, when the level difference portion passes the pulley, a problem that the power transmission belt abnormally vibrates and produces noise occurs.
FIG. 7 shows a V-ribbed belt as an example of a power transmission belt. In this figure, i indicates a sleeve made of elastic material such as rubber. The inner periphery of the sleeve is provided with a plurality of concavities j and convexities k for the formation of ribs.
To cope with the above problem, a power transmission belt and its producing method, as disclosed in Japanese Patent Application Laid-Open Gazette No.4-151048 (Prior Art 2), have been proposed in which both ends of a band-shaped fabric as a top fabric of the belt are butt-jointed, whereby an overlap of the top fabric which may cause deviation of the cord is eliminated so that no level difference can be produced at the jointed portion. This prevents the power transmission belt from abnormally vibrating and producing noise.
However, even if a level difference at the jointed portion of the top fabric can be eliminated by butt joint as proposed above, an adhesion rubber sheet to be wound around the top fabric in molding is generally lap-jointed at both ends thereof as in the case of the top fabric of Prior Art 1 belt. Accordingly, as shown in FIG. 8, in molding, the cord d is displaced in level in correspondence with the level difference of a lap-jointed portion m of the adhesion rubber sheet e, so that in the obtained power transmission belt the cord d is embedded into the adhesion rubber layer with deviated from an adequate position as in Prior Art 1 belt. As a result, when the transmission belt is used in serpentine drive, it may abnormally vibrate and produce noise.
In FIG. 8, n indicates a butt-jointed portion of the top fabric b. Reference is made to the figure where like reference signs refer to like parts as in FIG. 7.
Further, the butt joint of the top fabric is much hard and inefficient because the top fabric is thin, as compared with the case that a thick substance is butt-jointed. For this reason, I suppose that the lap joint of the top fabric has yet been generally made.